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Difficulty: EasyMeasures of Disease Frequency

An epidemiologist is reviewing surveillance data for tuberculosis in a specific municipality. On January 1, the municipality has a total population of 50,50050,500. At that time, 500500 individuals are already living with active tuberculosis. Over the next year, 100100 new cases of active tuberculosis are reported. Assuming a stable population with no migration or deaths, what is the incidence of active tuberculosis per 1,0001,000 at-risk individuals over this 11-year period?

Answer: 2 per 1,000

Answer

2
The incidence is calculated by dividing the number of new cases (100100) by the population initially at risk (50,00050,000), then multiplying by the desired multiplier (1,0001,000). The population at risk is the total population (50,50050,500) minus those who already have the disease (500500). Therefore, the incidence is 22 per 1,0001,000 at-risk individuals.

Step-by-Step Solution

1
Identify the number of new cases of the disease during the specified period.
100100 new cases
Incidence measures only the occurrence of new cases in a population over a given time.
2
Calculate the population at risk at the beginning of the period.
50,50050,500 (total population) - 500500 (existing cases) = 50,00050,000 individuals at risk
Individuals who already have the disease are not at risk of developing it and must be excluded from the denominator.
3
Calculate the incidence per 1,0001,000 individuals.
10050,000×1,000=2\frac{100}{50,000} \times 1,000 = 2
This formula normalizes the raw incidence to the requested per-1,0001,000 scale.

Key Concept

Incidence is defined as the number of new cases of a disease in a population at risk during a specified time period.
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Topics

Measures of Disease Frequency

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